Multilayer ceramic circuit boards have been used for many years for circuits for electrical apparatus, such as mainframe computers. Such ceramic circuit boards are made by casting glass and/or ceramic powders together with an organic binder and forming into tapes, called green tapes, upon which a metal circuit can be patterned. Vias are formed in each green tape that are filled with a conductive material so as to connect the circuits of the various layers electrically. The green tape layers are then aligned and stacked, pressed together, and fired to burn off organic residues and sinter the glass, forming a fired multilayer ceramic circuit board.
Originally ceramics such as alumina were used to form the green tape layers, but these ceramics require high firing temperatures, up to 1500.degree. C. This necessitated the use of refractory conductive metals, such as tungsten or molybdenum, to form the conductive circuit patterns. These refractive conductive metals can withstand high firing temperatures without melting. More recently, lower melting temperature materials have been used instead of alumina, such as devitrifying glasses that can be fired at temperatures of 1000.degree. C. or less. Multilayer ceramic circuit boards made of these glass materials can be used with lower melting point and higher conductivity metals, such as silver, gold or copper. However, these ceramic circuit boards have the disadvantage that they are not as strong as alumina based circuit boards.
Thus more recently, the multilayer ceramic circuit boards have been bonded onto metal or ceramic support substrates that are thermally conductive. The support substrate, which can be of a metal such as nickel/cobalt/manganese alloy or Invar.sup..RTM., a Feronickel alloy or a composite of Cu/Kovar/Cu, Cu/Mo/Cu or Cu/Invar.sup..RTM. /Cu and the like, or of a ceramic such as aluminum nitride, silicon carbide, and the like, imparts added strength to the composite board. However, there is a large mismatch of the coefficient of thermal expansion between conventional multilayer ceramic substrates and these support substrates. Support substrates made of metal for example do not shrink at all during the firing step, whereas the green tape layers used to form the ceramic substrate shrink about 20% in each dimension. Thus a bonding glass, such as described in U.S. Pat. No. 5,277,724 to Prabhu, has been used to adhere the green tape laminate to the support substrate. In addition, if chosen correctly, the bonding glass can totally suppress shrinkage of the green tape laminate on firing with respect to the support substrate in at least the two lateral, x and y, dimensions. Thus all of the shrinkage occurs in the thickness, or z, dimension only. This in turn reduces problems of alignment of the circuit patterns in the ceramic layers and contacts and via holes in the support substrate after firing. Thus multilayer ceramic substrates bonded to support substrates have become the medium of choice.
Conventional conductive via inks for the via connections between the multiple green tape layers are made by admixing an organic vehicle, a conductive metal powder, as of silver, gold, copper, alloys and mixtures thereof and the like, with a glass powder, generally of the same glass as is used to make the green tape. In such case the shrinkage characteristics of the green tape and of the glass in the vias are similar, and the glass in the via hole and the glass of the green tape readily sinter together at similar firing temperatures. Since the glasses and the shrinkage of both the green tapes and the conductive via fill inks are similar, the glass of the conductive via fill ink and that of the green tape layers sinter during the firing step, forming a gap-free interface.
However, in the case where a support substrate and a bonding glass layer are employed as part of the package, the shrinkage of the green tape layers in the x and y lateral directions is suppressed, and most of the shrinkage occurs in the z or thickness direction. We have found that this shrinkage behavior has an adverse effect on the integrity of conductive vias after firing the above support composite/multilayer ceramic circuit boards. Since the via holes are quite small, and the glass forms only a minority of the total conductive metal-glass composition of the via fill ink, the amount of glass in the via hole is small. In effect, this glass must shrink about 50% in volume in the thickness direction to maintain the same shrinkage characteristics as that of the green tape. Further, since the conductive metal powders which make up the majority of the conductive via fill ink formulations generally sinter before the glass-ceramic green tape compositions, the via fill inks tend to pull away from the walls of the vias during firing. This of course leads to discontinuities in the conductor paths at via-circuit intersections, and to non-hermetic vias.
The addition of more glass to conductive via fill inks has been tried in an effort to alleviate this problem and to permit more shrinkage of the via fill ink during sintering, but this solution leads to other problems, such as via bumps, cracking of the glass-ceramic around the vias as they are cooled, via porosity and even to vias that are not electrically conductive.
Thus an improved conductive via fill ink is required to overcome the above problems and to form conductive vias of integrity and correct shrinkage characteristics in ceramic multilayer circuit boards bonded to support substrates.